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One hundred not outBY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 30, 2008

The impact event that rocked the unsuspecting Siberian outback 100 years ago today may hold the secrets to the protection of our precious blue planet against a similar cosmic assault in the future.

As the sun rose on the 30th June 1908 in Central Siberia, it was drowned out by a bright light streaking across the sky. Moments later, the atmosphere was reeling from an almighty explosion that, according to eye witness reports, sounded a lot like persistent artillery fire. However, the Earth was not under attack from UFOs, as one popular theory insisted, neither had a black hole just passed through the Earth or antimatter been annihilated as equally popular hypotheses suggested, but instead the natives of the remote Tunguska region had just experienced the most powerful impact event in recent history: the explosion of an asteroid just a few kilometres above the Earth’s surface.

As the asteroid exploded, much of its energy was funnelled downwards towards the surface, say scientists at the Sandia National Laboratories. “There’s actually less devastation than previously thought, but it was caused by a much smaller asteroid,” says Mark Boslough. “That such a small object can do this kind of destruction suggests that smaller asteroids are something to consider.”

Felled trees as seen by one of the first expeditions to the Tunguska blast area. Image: University of Bologna, http://www-th.bo.infn.it/tunguska/.

It was fortuitous that the explosion occurred over such a remote, desolate region of the globe - it is not thought that any human lives were lost - however, had the asteroid arrived just a few hours earlier, it would have occurred over St Petersburg and we would have been taught an even crueller lesson about the vulnerability of our planet as it ploughs through such a rocky minefield. As it was, the blast felled around 80 million trees over an area of two thousand square kilometres and caused such dramatic atmospheric effects that Londoners could read their newspapers by natural light, all through the night.

But the energy of the blast was still not focused enough to gouge out a crater, or was it? Researchers from the University of Bologna think they have found the Rosetta stone of the Tunguska impact in the form of Lake Cheko. They are trekking out to the lake this summer to find out what was making an unusual signature in their seismic surveys they performed several years ago, a signature that they hope could be fragments of the elusive impactor. Although many impact experts are impressed with the team’s dedication, some are sceptical that the lake is associated with a typical impact event because it does not share certain characteristics with other well-studied impact craters.

“The impact cratering community does not accept structures as craters unless there is evidence of high temperatures and high pressures,” says impact cratering expert Gareth Collins of Imperial College London. “That requires evidence of rocks that have been melted or rocks that have been ground up by the impact.”

3D reconstruction of Lake Cheko based on real topographic data. The water level has been placed 40 metres below the actual level to show the main morphological features of the lake. Image: http://www-th.bo.infn.it/tunguska/.

The lake is also missing a characteristic ‘flap’ of overturned material from inside the crater that all other known craters exhibit, and on top of that, it has a rather unusual funnel shape, as opposed to the more typical bowl-shaped morphologies that planetary scientists can readily identify as impact craters on Earth and other planets in the Solar System. However, these well-studied craters share one thing in common: they were formed by the direct hit of an asteroid or comet, and were travelling fast enough such that they simply didn’t ‘see’ any intervening atmosphere, if indeed there was one at all. While there is a chance that fragments from the Tunguska airburst event could have reached the ground, it is unlikely that they were travelling with enough speed to dent the surface in comparable ways to larger impacts. Indeed, with the dawn of advanced computer simulations, scientists were able to show that a solid, stony object 50 metres across would not be expected to reach the ground at all. Yet it can be difficult to accept that an impact event can occur without leaving a calling card on the surface.

Although Tunguska was the most powerful impact event recorded in recent history, it was not the only significant airburst event that Earth has endured. Several smaller events have been witnessed on an alarmingly regular basis, including several over Canada on separate occasions in the 1960s, and more recently, one over Italy in 1993, and two in 2002 over Russia and the Mediterranean Sea respectively. A close call occurred on 18 January 2000, when a five-metre diameter object exploded at an altitude of 20 kilmetres over Yukon in Canada causing a loud bang, a flash of light, a shower of fragments and an electromagnetic pulse that caused temporary loss of power transmission over the area.

And there have been plenty of near-misses too. At the end of January this year a 250-diameter asteroid sailed past the Earth within spitting distance of the Moon, providing astronomers with a free close up glimpse of such a large object encroaching on Earth territory. But the extra-terrestrial visitor was not a complete surprise; it had first been spotted in October 2007 by astronomers using the Goldstone 70-metre radar antenna, part of NASA’s Deep Space Network that is capable of tracking kilometre-sized objects up to 20 million kilometres from the Earth.

A radar image of asteroid 2007 TU24 taken on 28 January 2008, 12 hours before its closest pass. Image: Arecibo Observatory/Greenbank Telescope.

Initial observations of this particular asteroid – 2007 TU24 – revealed vital information about its rotation rate, shape and composition. If an asteroid was on a collision course with the Earth then knowing these parameters would be crucial in understanding ways to deflect it.

“If a small body is on course for an Earth collision, radar reconnaissance could quickly allow one to recognise this by distinguishing between an impact trajectory and a near miss, and would dramatically reduce the difficulty and cost of any effort to prevent the collision,” says Dr Steve Ostro, Project Scientist for the Goldstone Solar System Radar. Scientists think that a gentle nudge may be our best strategy for defending the Earth from a potential asteroid strike, unlike the more explosive approaches favoured by many Hollywood scriptwriters.

And because smaller asteroids approach Earth statistically more frequently than larger ones, we really should be making more effort at detecting smaller ones than we have until now. And since the revised understanding of explosion mechanisms of asteroids in our atmosphere has been accepted by the scientific community, any strategy for defence or deflection should take this into account. Rather worryingly though, there are no such defence strategies currently in place, and, according to a recent report by the Task Force on Potentially Hazardous Near Earth Objects, developing a successful mitigation system could take up to a human lifetime, around 70 years, to fully implement.

The Arecibo radio dish in Puerto Rico is crucial to the detection of near-Earth asteroids. Image: NAIC-Arecibo Observatory/NSF.

And in the meantime, millions of asteroids are tumbling around the Sun, many of which play cat-and-mouse with the Earth as they dance in and out of the Earth’s orbit. Two thousand asteroids greater than one kilometre in size have Earth-crossing orbits, while over 80 million are the size of a house, or larger. Although it is statistically low that any of these objects will intersect the Earth on their respective orbits, they do pose a very serious threat to civilisation. An object just one kilometre in size would be big enough to cause world-wide disruption to our climate. Smaller impacts, like the Tunguska event, could occur on timescales of between a few hundred to few thousand years, but could still have the potential to cause irreparable damage to cities the size of London, Washington or Moscow.

Arthur C Clarke was right when he wrote “Sooner or later it was bound to happen,” the opening line of his 1972 novel Rendezvous with Rama, in which mankind learns the hard way about the dangers posed by incoming asteroids. Incidentally, that line was referring to the Tunguska airburst, and Clarke’s solution to the threat was an asteroid search program named Project Spaceguard, which has since become the name of such a real life program, and the search continues. But, sooner or later, it will happen again. The Earth is one hundred years not out in terms of a repeat Tunguska-sized performance, but the next asteroid may already have Earth in its sights.

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